When manipulating rods and tubular goods, particularly in the petroleum industry, it is often required to disengage or “break” connections between successive lengths of rod or tubing, and to meet specific operating requirements when doing so. In other circumstances, the same threaded interconnections have to be engaged or “made up” very precisely.
Specific examples of these of needs arise with “sucker rods”, which are used in downhole strings in petroleum wells to power a reciprocating or rotary pump from a power source at the surface. Sucker rod connections in use today are predominantly based upon the three part design and standards set in 1926 by the American Petroleum Institute (API). These standards were set so that, under given stress and other conditions the operator could assemble a sucker rod string that would meet the applicable standards and have a long lifetime of usage. With a reciprocating pump installed at a downhole location, for example, properly engaged API couplings and rods assembled into a sucker rod string of many hundreds of rods are calculated to operate through millions of cycles before failure. The sucker rods must withstand the varying forces generated during cycling although of necessity they must be limited in diameter, as in the range of ⅝th inch to 1 and ½ inches. The operator, to save costs, uses the smallest diameter tubing that will suffice to provide the cross-sectional area needed for product flow, which is inherently reduced by the size of the interior sucker rod string. In other words, sucker rods cannot be intentionally overdesigned, so that the rod lengths, and the couplings between them, must meet stringent requirements governed mainly by the internal diameter of the production tubing they are to operate in.
The API-type sucker rod configuration has, for each sucker rod, a threaded region at each pin end, together with an adjacent “wrench flat” at which torque is applied to engage and disengage the adjacent threaded portion in the pin end from the coupling, or internally threaded sleeve, in which the rod end is received. A protruding shoulder on the pin end between the threaded terminal section and the wrench flat is so designed as to engage the end of the coupling to prestress regions of both the threaded connection and the coupling to selected levels, on makeup. This prestressing is accomplished by torquing the rod pin end relative to the coupling during makeup by a selected fraction of a turn past the hand tight rod shoulder to coupling end contact point. The coupling receives the opposite ends of two rods in this fashion, and the design is such that the coupling nominally has greater strength and life expectancy than the body of the rod itself. When a sucker rod string is withdrawn upon failure, a need for maintenance, or by reaching its allowable lifetime, the connections must be “broken” (disengaged), in a manner which does not affect the anticipated lifetime of the unit.
There are a number of factors, however, principally fatigue-oriented factors, which affect the useful lifetime of a sucker rod string, and these factors tend to center in the coupling region. Much consideration has been given to analysis of the problems and causes of failure, because of the costliness of failures and the limitation on sucker rod life that such failures impose. These analyses have shown that, with repeated cycling, minor physical imperfections in a coupling or adjacent rod segment tend to become stress riser points, and that the imperfections grow with time, leading to early fracture or other failure of the connection. Relative movement between the rod pin end and the coupling, as can arise with excessive or inadequate prestressing at the threaded and shoulder regions, also introduces relative displacements between parts that tend to grow with time. This is another frequent cause of fatigue life failures.
Clearly, economics dictates that sucker rod strings be useable for as many total cycles and as many make and break operations as feasible. Particular care is paid by operators to the steps of disengagement of sucker rods from couplings, but there are other inherent problems which have not heretofore been satisfactorily overcome. When two sucker rods joined by an intermediate coupling are to be disengaged, each sucker rod can be gripped at its wrench flat by a separate wrench, and torque can be applied to break the connection with one of the rods, dependent upon whichever sucker rod is less firmly engaged. This of course leaves the coupling still engaged to the other sucker rod, and this connection cannot be broken, usually, without applying several thousand foot pounds of torque to the connection. To accomplish this without marring the surface of the coupling, the general industry approach has been to hold the coupling fixed with a smooth-faced friction wrench, and then to attach a wrench with a notched end to the wrench flat area. Torque is then applied manually to the handle, or an extension of the handle, so as to break the connection free solely by muscle power via the extended lever arm. A smooth-faced friction wrench will typically not provide enough manual retention force without slipping, so that sand is often interposed between the coupling and wrench faces to increase frictional restraint, which is not always adequate and also may mark the coupling surface.
As pointed out in Clark U.S. Pat. No. 5,010,635, this manual procedure is time consuming and often dangerous, so Clark proposes a mechanical, power operated friction wrench design. This mechanical and presumably automatic machine has not found widespread use, however, because of various difficulties encountered in practice, because of a lack of versatility in accepting different sucker rod sizes, and because it is not intended for use in precision make up, but only in breaking connections.
The makeup of connections between sucker rod-coupling combinations of the type described in the above-referenced Carstensen application makes possible significant benefits in terms of load bearing capabilities and fatigue life. The displacements between the four components of the connection can be precisely controlled in a maimer which is highly advantageous for meeting operational requirements at the production or production workover rig. Operators heretofore have used a torque measurement method or a circumferential displacement method, or a combination of the two. In the torque measurement method the power tong driver unit is set at a given level or an indicator gives a reading of pressure or torque, while in the displacement method the angle of relative rotation between pin end and coupling is used. To do this the two components are marked with indicia which are to be brought into coincidence when torque is applied. Crews will often start with the circumferential displacement method and observe readings of how much torque is required, and thereafter follow the pressure or torque indicators instead of trying to match the displacement lines precisely, which is much more time consuming.
Extensive studies of pin end to coupling engagement, however, have shown that these prior techniques are subject to substantial variations in final relative positions, depending upon such factors as the relative wear of the thread forming tool, and the number of uses of the coupling or pin. Even though the elements are within tolerances, the actual engagement that results varies to an undesirable extent. In the novel sucker rod connection design disclosed in the Carstensen application, in which a torque washer alternative is used, the dimensions between the pin end shoulder and the end face of the pin end are precisely maintained. Also, the axial length of the coupling itself, and the axial length of the torque washer that is interposed between the end faces of the pin ends are precisely controlled. When these units are properly made up, the pin necks are prestressed to selected tensions, the coextensive portions of the coupling are under compression between the thread regions and the pin end shoulders, and the central torque washer is under compression while the coextensive length of the coupling at the center region is in tension. This arrangement provides a unified structural combination which greatly inhibits micro-movements of the elements relative to each other and locks the threads into firm engagement.